Horizontal bar railings are usually assembled at the site of installation by one of two methods. Either the railing members are assembled and bolted together, or the railing members are laid out and welded together. Both of these methods have significant drawbacks.
The bolting method of installation requires a great amount of manual labor to assemble the railing members, and is therefor costly. Further, the shear force from crowd impact, etc., imparted to the horizontal bars is usually borne only by the bolts received in the line posts, which sometimes prove to be insufficient for the task. Furthermore, mechanical assemblies tend to lossen with age and to rattle.
Welding is structurally more advantageous as an assembly method, but again it is costly in labor and energy. Also, an all-welded construction is often unable to accommodate thermal expansion and contraction, making it unsuitable for many types of applications.